Listen bro,
I understand your logic whole-heartedly. I think everyone here was a beginner at one point, I know I was. And I was broke and had no gear, so I get it.
However, the danger with fab'ing it, IMO, is that it is going to be hard for you to match the craftsmanship and quality control that goes into the port-a-wrap in your shop. That being said, I am sure that you can make something that would work. I had an excellent welder that worked for me when I was full-time in NY, and I would bet a case of beer that he could make me a port-a-wrap for less money than I could buy one.
HOWEVER:
Sooner or later whatever you make is gonna break. And when it breaks, it's gonna be at the most inopportune time. You could kill yourself, or someone else. You could destroy a house, crush a truck, break your leg, smash your groundie's head, etc etc etc. When, not IF, but WHEN your welds eventually fail, your going to have a real mess on your hands.
Save up the 200 bucks, and get the legit Port-a-wrap.
My two cents.
T